A boat whose speed in still water is 15 km/h goes 30 km downstream and comes back upstream in 4.5 hours.
What is the speed of water current?

Speed of the boat in still water b = 15 km/h.

Distance travelled downstream and upstream = 30 km each

Total time taken for both downstream and upstream journeys = 4.5 hours  

Formula Used: 

Speed = $$\frac{distance}{time}$$​​

Solution:   

Let speed of current be x km/hr

According to the question:

​$$\frac{30}{15+x} + \frac{30}{ 15-x} = 4.5 \\ \ \\ 30\left[ \frac{15-x + 15 + x }{(15+x)( 15-x)} \right] = \frac{45}{10} \\ \ \\ \frac{30 }{ (15)^2-x^2 } = \frac{3}{20} \\ \ \\ 200 = x^2 - 225 \\ x^2 = 25 \\ x = 5$$ 

Thus, the speed of current is 5 km/h.